1. Field of the Invention
This invention relates to an apparatus and method for the self-measurement of intraocular pressure.
2. Background Art
Intraocular pressure (IOP) is a physiological parameter routinely measured by eye care professionals. Elevated IOP is the most important risk factor in primary open angle glaucoma (POAG) which, combined with normal tension glaucoma (NTG), is the second leading cause of irreversible blindness in the United States. Patients with POAG and NTG have the same characteristic optic neuropathy (cupping) and visual field loss, but in NTG the IOPs have never been found to be elevated. Elevated IOP is also found in patients with ocular hypertension (OHT), but not the neuropathy or field changes. The only current treatment for POAG, NTG and OHT is reduction of IOP.
The instrument that is the reference standard for IOP measurement is the Goldmann applanation tonometer, used worldwide by opthalmologists for over 40 years. This instrument functions to flatten part of the cornea to measure eye pressure, wherein the pressure within the eye is determined by how much force is needed to flatten the cornea.
Glaucoma management, which is so dependent on IOP, would benefit greatly by the acquisition of more IOP data. Essentially all IOP measurements are obtained on visits to the opthalmologist's office—usually one measurement during typical office hours, and rarely more than one visit every two or three months. In glaucoma management, there is no parallel to the ubiquitous monitoring by diabetic patients of capillary blood glucose or by arterial hypertensive patients of blood pressure and heart rate. For these conditions, adjuncts in patient care increase the volume of measurements during clinic hours as well as extend the monitoring beyond the eight hours that the clinic is open.
Measurement of IOP at different times of the day usually yields different readings, sometimes highest at night. However, there is considerable variability in the diurnal pattern between individuals. Differences in IOP throughout the day are of special interest. In some POAG patients, despite treatment which results in normal IOPs (measured in the opthalmologist's office), cupping and field loss can progress. In NTG, cupping occurs and can progress in the presence of IOP within the normal statistical limits (measured in the opthalmologist's office). In OHT, over time, cupping and field loss can develop. The question in these cases is whether the progression (in POAG and NTG) and development (in OHT) of glaucoma damage is due to elevated IOP at times of the day when they cannot be measured in the opthalmologist's office.
The answer is a clinical test with a long history, the diurnal IOP curve, which involves measuring a patient's IOP a number of times throughout a 24 hour period. In “Emerging Perspectives in Glaucoma: Optimizing 24-hour Control of Intraocular Pressure” (Am J Ophthalmol 2002, 133: S1-S10), Wax et al. summarize the importance of 24-hour control of IOP in the management of POAG and NTG to prevent patients from progressing to blindness (see also Oliver et al., Am J Ophthalmol 2002, 133: 764-772). Perhaps in OHT, in which standard medication protocols reduce the incidence of progression to cupping and visual field loss, an additional risk predictive factor might be uncovered in this inhomogeneous group by expanding the scope of IOP testing from an 8- to a 24-hour day.
However, the diurnal IOP curve is a problematic test because it typically involves admitting the patient to a hospital or sleep laboratory where a resident or technician measures IOP at intervals throughout the day and night. It seems likely that results of diurnal curves might be affected by the inherently more stressful institutional setting, sleeping in an unfamiliar bed in a strange hospital room or sleep laboratory, and being awakened multiple times during the night by someone who measures the patient's IOP. In one systematic study of diurnal IOP using the Goldmann tonometer (see Hayreh et al., Am J Opthalmol 1994, 117: 603-624), the earliest measurement was at 7 am and the latest was at 10 pm. Another study reported Goldmann readings throughout the night, sitting and “10 meters” from the patient's room (see Ido et al., Opthalmol 1991, 98: 296-300). This study showed that frequent awakening of the patient in a hospital for the measurement can be a confounding factor, and so the research design was altered to awaken the patient once at night at a random time. Therefore, obtaining a full diurnal curve with this protocol would require the patient to be admitted to the hospital or a sleep laboratory four to five different nights, ideally with a slit lamp with a Goldmann tonometer in the patient's room to measure IOP immediately upon awakening while in the lying position. Of course, this is not possible economically and logistically for in-patient care or screening. In reality, diurnal IOP curves are currently not generally part of the standard of care in glaucoma management, except in clinical research centers. When diurnal curves are obtained, data are typically limited to several points during a single, likely uncomfortable, night.
In obtaining diurnal IOP curves, the tonometric method is an important, but not a simple, consideration. Because of the complicated logistics, this test has often been done without using the Goldmann applanation tonometer. For example, recently, Liu et al. reported that the lying position is a factor in the increase in IOP in some patients, although the nighttime values in the lying position were not compared with sitting nighttime IOP measurements (Invest Opthalmol Vis Sci 1999, 40: 2912-2917). This extensive study was based on measurements made with a pneumotonometer, which has been shown to correlate well with the Goldmann applanation tonometer (see Quigley and Langham, Am J Opthalmol 1975, 80: 266-273). However, the pneumotonometer is an instrument on which opthalmologists do not base their clinical decisions.
The effect of a subject's body position on IOP has been the source of much debate in the literature. Of the many daytime studies, most using the Goldmann tonometer, most have shown a 1-4 mm Hg higher pressure in the lying position (see Tsukahara and Sasaki, Br J Opthalmol 1984, 68: 389-392; Yamabayashi et al., Br J Opthalmol 1991, 75: 652-655; Anderson and Grant, Invest Opthalmol 1973, 12: 204-212), some a larger difference (see Leonard et al., Br J Opthalmol 1983, 67: 362-366), and some no difference at all (Frampton et al., Am J Optom Physiol Opt 1987, 64: 54-61; Strobl and Follman, Ophthalnologica 1962, 144: 57-61; Kindler-Loosli et al., Albrecht v. Graefes Arch klin exp Opthalmol 1975, 194: 17-21). There has been no study of nighttime Goldmann IOP in patients in the lying position. In addition to position, other factors have been reported to influence a patient's IOP throughout the night, including the light that a patient's eyes receive (see Frampton et al.), the blood melatonin level (see Willdosoet et al., Ophthal Physiol Opt 1993, 13: 357-365), a blood pressure change associated with waking (see Zeimer et al., Opthalmol 1990, 97: 547-550), the fact that the patient has or has not slept (see Frampton et al.; Brown et al., Ophthal Physiol Opt 1988, 8: 246-248; Brown et al., Ophthal Physiol Opt 1988, 8: 249-252), and the actual state of sleep the subject was in when awakened (see Noel et al., Opthalmol 2001, 108: 139-144). This body of research makes it seem unlikely that a higher IOP at night, when it occurs, is due entirely to position.
The disadvantages for both patients and medical personnel of an institutional site in measuring diurnal IOP led to the idea of home tonometry, which Posner noted in 1965, having patients use a Maklakoff type tonometer (Eye & Ear Nose Throat Mon 1965, 44: 64-66). Jensen and Maumenee (Am J Opthalmol 1976, 76: 929-932) and later Alpar (Glaucoma 1983, 5: 130-132) had a family member measure the patient's IOP with the Schiotz tonometer.
A more recent approach to measuring diurnal IOP in the home environment introduced the concept of self-tonometry. Two technically sophisticated instruments, both hand-held and based on the applanation principle of the Goldmann tonometer, have been studied. In Zeimer and Wilenski's instrument (IEEE Trans Biomed Eng 1982, 29: 178-183), the IOP endpoint is detected by a photodiode array optical device instead of the signature pattern recognition used in Goldmann tonometry. Draeger and group used a microprocessor controlled optical sensor (see Groenhoff et al., Int Opthalmol 1992, 16: 299-303). Both showed promise in the hands of their inventors, but others have found the correlation of patient measurements and ophthalmologist measurements using the Goldmann tonometer problematic, and also found that these devices can be moderately difficult to use. What may be most significant is the limited interest in these instruments since their invention in the 1980's, despite the concurrent heightened awareness of the potential importance of diurnal IOP.
As would be expected, self-tonometry with non-contact tonometers (see Stewart et al., Ann Opthalmol 1991, 23: 177-182; Carenini et al., Int Opthalmol 1992, 16: 295-297) that have been shown to be less reliable than the Goldmann method in the hands of opthalmologists has met with a general lack of professional interest. Finally, the Tono-Pen®, based on the McKay-Marg applanation principle, is used by some opthalmologists' technicians for IOP screening. While it has occasionally been used for self-tonometry (see Kupin et al., Am J Ophthalmol 1993, 116: 643-644), it is not easy to apply to oneself, and an ophthalmologist would not depend on measurements with a screening instrument as a basis for clinical decisions.